The present invention relates, in general, to electromagnetic acoustic transducers and inspection systems utilizing same and, in particular, to a new and useful fast reacting ground fault interrupter for use with high current pulsers for driving electromagnets used with electromagnetic acoustic transducers (EMATs).
Ground fault interrupter (GFI) circuits are commonly available (and for certain circuits in household wiring are required) protection circuits for standard power distribution applications. Generally these devices sense the difference between the current flowing in the high side (supply side) and low side (return) of a circuit, with the low side of the circuit connected to safety ground only at the power source. Normally, the current in the high side of the circuit equals the current in the low side of the circuit so that the difference is zero. When a short or leakage path develops between the circuit and safety ground, some of the circuit current flows through the safety ground to return to the low side of the power source, causing the current in the high side and low side of the circuit to become unequal. The ground fault interrupter (GFI) senses this difference in currents and shuts off the circuit. The devices that are known to the inventors operate with either AC (alternating current) or DC (direct current) power circuits.
EMATs are the basis of a noncontact ultrasonic inspection method that requires no fluid couplant because the sound is produced by an electromagnetic interaction within the material. The Babcock & Wilcox Company has developed a high current, transistor switched magnet pulser circuit for use with EMATs. This pulser circuit, in conjunction with a pulsed electromagnet, produces the magnetic field necessary for the operation of EMATs in many applications. The pulser circuit is capable of producing 200 amp pulses at supply voltages of up to 300 V DC. In operation, the pulser circuit connects a DC power source to the terminals of the pulsed magnet using transistor switches. Current ramps up in the pulsed magnet, limited by the resistance and inductance of the pulsed magnet. After a sufficient amount of time (typically several hundred microseconds) the current and therefore the magnetic field reach a level required for the operation of the EMAT. At this time an ultrasonic signal is launched and then received by the EMAT. The transistor switches are then turned off, shutting off the current flow into the pulsed magnet. By using this pulsed operation large magnetic fields are produced with a much smaller electromagnet and much less power dissipation than a DC electromagnet. Typical rising current pulse lengths are 0.1 to 5 milliseconds.
Transistor switched magnet pulsers frequently produce high-voltage and/or high-current pulses. The high voltages and high currents can present personnel safety hazards to people in contact with the magnets or associated structures in the event of a fault in the magnet coil insulation or magnet cable insulation. In addition, high fault currents may result in damage to the transistor switched magnet pulser circuit itself, or the component being tested if the component is grounded and is in contact with the pulsed magnet or supporting structure.
Accordingly, it has become desirable to provide a ground fault protection circuit for operation with EMAT transistor switched magnet pulsers to quickly shut off the transistor switched magnet pulser when a ground fault in the pulsed magnet or connecting cable is detected.